This invention relates generally to removal of oxides of nitrogen and of sulfur from combustion processes; more particulary, it concerns unique and highly efficient apparatus to secure such contaminant removal.
Sulfur oxides (SOx) and nitrogent oxide (NOx) are the two most important atmospheric pollutants in the United States. About 75 percent of the SOx emissions in this country come from stationary combustion sources, primarily utility and industrial boilers. The remaining 25 percent comes from smelting copper, lead, and zinc sulfide ores, and from other industrial processes. About 98 percent of the SOx emissions are due to SO.sub.2 with the remaining 2 percent due primarily to SO.sub.3. NOx emissions are produced by all combusion processes that use air as the source of oxygen, even methane burners.
The primary source of SOx emissions are sulfur-bearing coals which range from about 0.3 to 8.0 percent sulfur by weight in the United States. This sulfur comes in either pyritic or organic form, and can be separated from coal by various physical means, especially water washing. However, organic sulfur is chemically linked to coal, and chemical means are required for its removal.
Most commercial installations currently use wet, nonregenerative lime/limestone flue gas desulfurization (FGD) procedures. These procedures produce calcium sulfite or calcium sulfate sludges which require disposal. Typically about 85 percent of the SO.sub.2 and 99 percent of the particulates are removed by FGD. In non-regenerative scrubbing processes, solids disposal is a major problem. The mass of solids per unit mass of sulfur removed ranges from 4.0 to 6.6. Therefore, the large solids-waste handling and disposal problem has encouraged and made necessary efforts to develop alternative procedures for the removal of SO.sub.2.
Nearly every combustion process that uses air as the source of oxygen produces NOx pollutants. At high temperatures, the atmospheric nitrogen combines with oxygen to form NOx compounds. Nox is most commonly emitted as either NO or NO.sub.2, the NO being readily oxidized in the atmosphere to NO.sub.2.
The most common NOx emission control strategy in the United States is to modify the combustion process to limit the flame temperature, low temperature inhibiting the rate of formation of NOx compounds. SOx scrubbers also remove some of the NOx emissions. Selective catalytic reduction (SCR) has been widely used in Japan for NOx reduction, mostly on oil-fired boilers, but on some coal-fired boilers as well. System suppliers routinely design and guarantee the SCR reactor for 80 percent NOx removal. SCR technology is not solely relied on for NOx reduction, but is usually combined with combustion modifications.
There is need for improved systems and methods to overcome problems and difficulties with the above techniques, including problems with solids disposal, solids waste-handling, and re-heat steps required with existing wet scrubbing FGB systems. Also, there is need for apparatus and process according to the present invention wherein SOx and NOx are removed from combustion gas streams, and essentially pure sulfur pure nitrogen is released to atmosphere.